A drill having a releasably mounted cutting head is disclosed in U.S. Pat. No. 5,228,812. In a first embodiment disclosed in U.S. Pat. No. 5,228,812 the drill has an insert portion for cutting a workpiece and a shank portion, the insert portion being separably mechanically connected to the shank portion. The insert and shank portions have mutually engaging portions forming the connection between the insert and the shank, and a slit formed in either the insert or shank engaging portion. The insert portion and the shank portion are connected with each other using an elastic force which is caused by an elastic deformation upon mutual movement of the opposite surfaces of the slit in an engaging state of both portions, whereby the insert portion may be connected with or fixed to the shank portion by press-fitting the insert portion into the shank portion. In the connected state, the insert is fixed to the shank by frictional force between the side portions of the held portion of the insert and the inner end surfaces of holding portions of the shank.
The drill disclosed in U.S. Pat. No. 5,228,812 is often called a spade drill which is characterized by an insert portion (i.e., the cutting portion, or cutting head) having a basically flat shape with two opposing parallel flat surfaces which is received in a corresponding flat gap formed in the shank. The insert portion and shank portion are brought into engagement by relative axial displacement of the two components as they are brought together. The insert portion is made of a hard material such as a surface coated cemented carbide and the shank is made of steel. In order to insure a required accuracy of the opposing parallel flat surfaces, the flat surfaces may be ground. The grinding operation requires two grinding operations, one when grinding one flat surface, and another when grinding the other flat surface. Between the two grinding operations, the insert portion has to be rotated by 180° in order to present the non-ground flat surface to the grinding wheel, which may introduce an inaccuracy in the positioning of the non-ground flat surface relative to the grinding wheel.
U.S. Pat. No. 7,306,410 discloses a rotary cutting tool, e.g. a twist drill, having a tool head made of a hard cutting material, e.g. tungsten carbide, and a tool shank made of a high-speed tool steel or another metal material. The tool head has a fixing peg that is coaxial with the axis of rotation of the tool and which is in the shape of a truncated cone that tapers toward the tool shank with the axis of rotation as the cone axis. Two spiral flutes divide the surface of the fixing peg into two truncated conical generated surfaces that lie essentially diametrically opposite each other. The tool shank clamps the replaceable tool head in position on its holding end between the facing flanks of two clamping extensions which project essentially in the axial direction from the tool shank. The conical generated surfaces of the fixing peg are, in the clamping position, pressed by the facing flanks of the two clamping extension of the tool shank. The inner flanks which face each other of the clamping extensions are realized in a concave shape corresponding to the convex truncated cone generated surfaces of the fixing peg, so that in the fixing position they press with their entire surfaces against the truncated cone generated surfaces of the fixing peg.
In an exemplary embodiment illustrated in U.S. Pat. No. 7,306,410 a clamp fixing screw screwed in from the peripheral side of the tool shank runs through the fixing peg and the clamping extensions that clamp it in position. The clamp fixing screw not only represents a positive-fit connection between the clamping extensions and the fixing peg, but also increases the clamping pressure which is exerted radially by the clamping extensions on the truncated cone generated surfaces of the peg. In an exemplary embodiment, the clamp fixing screw lies inside a slot, the axis of rotation of which runs diametrically, and which slot runs radially inside the fixing peg and is open toward the tapered end of the fixing peg. The requirement of a clamp fixing screw introduces an extra complication and longer set up time when securing the replaceable tool head to the tool shank.